Design of a Low-Noise Photon-Count Readout ASIC for Si-PIN-Based Personal Dosimeters With Low Energy Detection Threshold

A portable electronic personal dosimeter (EPD) with a battery supply is widely used to monitor the dose equivalence of staff in nuclear radiation environments, requiring both a low energy detection limitation and a long operation period. Recently, a commercial silicon p-i-n (Si-PIN) detector has been proposed as a radiation sensor due to its advantages of low cost and high sensitivity. However, such a detector suffers from a large parasitic capacitance, i.e., 20 pF at a supply of several tens of volts. Thus, designing a low-noise front-end readout application-specific integrated circuit (ASIC) at low power dissipation is very critical. In this article, we propose design techniques and characterization of a fully customized low-noise ASIC for this application. An improved charge-sensitive amplifier (CSA) and a CR-(RC)3 pulse shaper with programmable shaping time are proposed to achieve both a low noise floor and a smaller slope of equivalent noise charge (ENC) at room temperature. A four-channel ASIC has been successfully developed in the $0.35~\mu \text{m}$ complementary metal oxide semiconductor (CMOS) process. The die size of the prototype chip is 2.5 $\times $ 2.5 mm. The dynamic range of input charges is 0.2–15 fC. The ENC is 102 + 3.5 e− (rms) per picofarad under a power dissipation of ~2 mW/channel. Through the measurement of the ASIC connected to a Si-PIN detector, the energy resolution is 3.8% full-width half maximum (FWHM) at the 59.5 keV of the radioactive source 241 Am. The energy detection threshold is less than 5 keV and the maximum count rate is 320 kcps. The proposed ASIC has passed the verification of a stereotyped EPD.